1. Field of the Invention
The present invention relates to a multilevel light-intensity modulating circuit (or apparatus) for producing a multilevel modulated optical signal.
2. Description of the Related Art
In accordance with the requirements for increasing the transmission capacity in optical communication systems, improvement of the efficiency of using optical bandwidth is required in wavelength-division multiplexed transmission systems, thereby producing an important goal of performing band suppression of the optical spectrum so as to arrange a plurality of wavelength channels at narrow spacing.
In order to achieve this goal, multilevel (or multivalued) signals are used so as to decrease the bit rate, thereby suppressing the spectrum width. For example, in comparison with conventional methods, such as two-level light-intensity modulation methods, when 2n amplitude levels are defined in signal transmission, the same amount of data (as that transmitted in the conventional method) can be transmitted at a bit rate of 2/2n and the spectrum width can also be suppressed to approximately 2/2n as much as the band necessary for the conventional methods.
FIG. 7 shows an example of the structure of the conventional multilevel (here, four-level) light-intensity modulating circuit, where the light source 66 is not included in the modulating circuit (refer to S. Walkin et al., xe2x80x9cA 10 Gb/s 4-ary ASK Lightwave Systemxe2x80x9d, ECOC 97, Conference Publication No. 448, pp. 255-258, 1997).
In the figure, two two-level (or binary amplitude-shifted) electric signals having the same power are respectively input into two input terminals 61 and 62. The power of one of the two electric signals is attenuated to half by using the attenuator 63, and two signals are then combined by the power combiner 64, thereby producing a four-level (or quaternary amplitude-shifted) electric signal. This four-level electric signal is applied to the light-intensity modulator 65, in which the intensity of an optical carrier output from the light source 66 is modulated, thereby producing a four-level modulated optical signal.
FIGS. 8A to 8D show eye patterns which can be observed in a numerical calculation for producing a four-level electric signal by using two two-level electric signals, and further producing a four-level modulated optical signal.
That is, two two-level electric signals, whose eye patterns are respectively shown in FIGS. 8A and 8B, are electrically combined using a system as shown in FIG. 7, so that a four-level electric signal as shown in FIG. 8C is produced. This signal is used for intensity modulation of the optical carrier, thereby producing a four-level modulated optical signal as shown in FIG. 8D.
Here, a Mach-Zehnder light-intensity modulator is commonly used as the light-intensity modulator 65. FIG. 9A is a diagram showing the response characteristics obtained when the Mach-Zehnder light-intensity modulator is used for two-level intensity modulation. As is clearly shown by the figure, amplitude distortion in each level of mark xe2x80x9c1xe2x80x9d and mark xe2x80x9c0xe2x80x9d in the two-level electric signal is suppressed, that is, a two-level modulated optical signal having preferable characteristics is obtained.
However, when intensity modulation of the optical carrier is performed using a four-level electric signal, amplitude distortion at level xe2x80x9c0xe2x80x9d and level xe2x80x9c3xe2x80x9d is suppressed, but amplitude distortion at levels xe2x80x9c1xe2x80x9d and xe2x80x9c2xe2x80x9d is increased, as shown in FIG. 9B.
In addition, the response characteristics of the Mach-Zehnder light-intensity modulator is non-linear; thus, in order to equalize each interval between adjacent levels in the four-level modulated optical signal, a four-level electric signal, in which the interval between levels xe2x80x9c1xe2x80x9d and xe2x80x9c2xe2x80x9d is narrowed in advance, must be produced. This condition is also required when intensity modulation is performed using a multilevel (more than four-level) electric signal, and it is inevitable to suitably define the interval between the intermediate levels, and amplitude distortion should be suppressed.
It is desirable, therefore, to provide a multilevel light-intensity modulating circuit for suppressing the amplitude distortion regarding intermediate levels, caused by the conversion from a multilevel electric signal to a multilevel modulated optical signal.
The present invention provides a multilevel light-intensity modulating circuit comprising:
an optical distribution section for distributing an input optical carrier into n-channel optical carriers, where n is an integer of 2 or greater;
n light-intensity modulators into which the n-channel optical carriers are respectively input, wherein each light-intensity modulator modulates intensity of the input optical carrier by using an input two-level electric signal and outputs a two-level modulated optical signal;
an optical phase control section for producing a phase difference between the n-channel two-level modulated optical signals which are respectively output from the n light-intensity modulators;
a light-intensity control section for assigning a different light intensity to each of the n-channel two-level modulated optical signals which are respectively output from the n light-intensity modulators; and
an optical coupling section for combining the n-channel two-level modulated optical signals obtained via the optical phase control section and the light-intensity control section, and outputting a 2n-level modulated optical signal, wherein:
the phase difference produced by the optical phase control section and the different light intensity assigned by the light-intensity control section are defined in advance so as to produce the 2n-level modulated optical signal.
The optical phase control section may be positioned at the input or output side of at least one of the n light-intensity modulators.
The light-intensity control section may be positioned at the input or side of at least one of the n light-intensity modulators.
As a typical example, the light-intensity control section has a structure for respectively attenuating the light intensities of (nxe2x88x921) channel input signals to 1/2, 1/4, . . . , 1/2nxe2x88x921 as high as the original light intensities.
In a specific example,
n=2;
the optical distribution section has a distribution ratio of 1:1;
the light-intensity control section defines the light-intensity ratio between the 2-channel modulated optical signals as 2:1xc2x18%;
the optical phase control section provides a phase difference of 90xc2x0xc2x13% between the 2-channel modulated optical signals; and
the optical coupling section has a structure for coupling the 2-channel modulated optical signals and producing the four-level modulated optical signal.
The present invention also provides a multilevel light-intensity modulating circuit comprising:
an optical distribution section for distributing an input optical carrier into n-channel optical carriers, where n is an integer of 2 or greater;
n light-intensity modulators into which the n-channel optical carriers are respectively input, wherein each light-intensity modulator modulates intensity of the input optical carrier by using an input two-level electric signal and outputs a two-level modulated optical signal;
an optical phase control section for producing a phase difference between the n-channel two-level modulated optical signals which are respectively output from the n light-intensity modulators; and
an optical coupling section for combining the n-channel two-level modulated optical signals obtained via the optical phase control section, and outputting a 2n-level modulated optical signal, wherein:
a distribution ratio of the optical distribution section, a coupling ratio of the optical coupling section, and the phase difference produced by the optical phase control section are defined in advance so as to produce the 2n-level modulated optical signal.
The optical phase control section may be positioned at the input or output side of at least one of the n light-intensity modulators.
In a specific example,
n=2;
the optical distribution section has a distribution ratio of a:1;
the optical coupling section has a coupling ratio of b:1, where axc2x7b=2xc2x18%;
the optical phase control, section is positioned at either of input and output sides of one of the two light-intensity modulators, and the optical phase control section provides a phase difference of 90xc2x0xc2x13% between the 2-channel modulated optical signals; and
the optical coupling section has a structure for coupling the 2-channel modulated optical signals and producing the four-level modulated optical signal.
Either of the above-explained multilevel light-intensity modulating circuits may be integratedly formed on a lithium niobate (LN) substrate, wherein each light-intensity modulator is a Mach-Zehnder light-intensity modulator.
According to the present invention, a plurality of two-level modulated optical signals are produced using two-level electric signals which respectively correspond to the modulated optical signals, and the phases and light intensities of the produced two-level modulated optical signals are controlled so as to combine the optical signals, thereby producing a multilevel modulated optical signal. Accordingly, the suppression of amplitude distortion at marks xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d of the two-level modulated optical signals is effectively used for suppressing the amplitude distortion at all levels of the multilevel modulated optical signal.
Japanese Unexamined Patent Application, First Publication No. Sho 63-5633 (xe2x80x9cOptical Multivalued Communication Systemxe2x80x9d) discloses a conventional system in which a plurality of two-level optical signals are produced using different light sources, and the produced two-level optical signals are combined. However, the optical phase relationships between the different light sources are random. In this case, the phase control cannot be substantially performed, thereby producing interference noise when the optical signals are combined.
In contrast, in the present invention, a single optical carrier is divided so as to produce a plurality of two-level optical signals; thus, each phase difference between the optical signals is fixed. Therefore, each phase difference can be controlled, so that no interference noise is generated when the optical signals are combined. In addition, the optical phase difference and the light-intensity ratio between the optical signals can be set to suitable values in advance, thereby minimizing the amplitude distortion and equalizing each level interval.
As explained above, the system disclosed in Sho 63-5633 requires a number of light sources corresponding to the number of multilevels; therefore, the temperature and electrical power must be controlled for each light source, so that the control is complicated and space for installing each light source is necessary. In contrast, the present invention requires only a single light source regardless of the number of multilevels, so that the system structure and control for the multilevel light-intensity modulating circuit can be simplified.
In addition, in the above conventional system, direct modulation is performed; thus, chirp (transient variation in optical wavelength) occurs in accordance with increase of the modulation speed. In contrast, the present invention performs external modulation, so that it is difficult for chirp to occur, and it thus can be applied to high-speed modulation.
On the other hand, in the conventional system disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 10-209961, when the electric signal has amplitude distortion, the distortion is projected onto the output optical signal and also grows (refer to the above-explained xe2x80x9cDescription of the Related Artxe2x80x9d). In contrast, in the present invention, even when the electric signal has amplitude distortion, it is possible to produce an output multilevel signal in which the distortion is suppressed.